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When Cyril Houri and a group of friends he refers to as GPS alpha geeks were frustrated by poor satellite reception in the big cities where they lived, they decided to do something about it. The result is Navizon, a wireless positioning system that works on Pocket PC PDAs by triangulating signals from Wi-Fi access points and GSM cellular towers. The company hard-launched today after three months of testing with a network of 5,000 users.

Its like a software-only virtual GPS, says Houri, president of Mexens Technology LLC, the New York-based company he launched earlier this year to develop and market Navizon. Its also a little similar to peer-to-peer file sharing. One person buys a GPS [device], maps his city and creates a virtual GPS. Then everybody else can benefit from it.

Houri has been around geopositioning for awhile. His first company, Infosplit, developed a positioning system based on using IP addresses. He sold it in March 2004 to competitor Quova Corp. Navizon offers much more precise positioning.

Heres how it works. You load the software on a Pocket PC with built-in GPS and, ideally, both Wi-Fi and cellular phone functionality. Then, as Houri says, you just live your normal life.

As you walk around your city, the device receives broadcast signals from Wi-Fi access points  hotspots, home Wi-Fi networks, company WLANs (much like the competitive service built by Skyhook Wireless), but Navizon adds cell towers to the mix. The Navizon software takes signal strength measurements from the built-in Wi-Fi and/or cellular radios  at three different locations for each AP or tower detected. Since Navizon knows its own location from the Pocket PCs GPS, the readings of signal strength are enough, using sophisticated algorithms, to triangulate the position of each AP or tower. The software then records those locations. The Navizon software does all of this automatically.

Once the software has built a database of local AP and tower locations, it can accurately calculate its position by triangulating from three or more of them  again by measuring signal strength and applying proprietary algorithms. It works even when GPS doesnt, including inside.

So Navizon solved the problem Houri and his fellow GPS geeks were originally grappling with  it complements and increases the accuracy of GPS in built-up areas where coverage is otherwise often poor. They took it a step further by emulating, at least conceptually, what peer-to-peer systems do. Navizon makes available the database of AP and tower locations built by GPS-equipped users  free  to users who dont have GPS.

Houri is quick to point out that in one respect the parallel with peer-to-peer file sharing doesnt apply. It stops at the legality, he says. What we do is 100% legal. The Navizon software doesnt try to gain access to Wi-Fi networks through the APs it maps, it only has to receive the anybody there? messages that most continuously broadcast to make it easier for legitimate users to access the network.

Navizon does work like peer-to-peer systems in other respects. When a GPS-equipped user cradles his Pocket PC, the Navizon software synchronizes with a central server, uploading the new AP and tower locations he has mapped, and downloading new locations others have mapped for his local area  which is whatever area hes in, but usually his home area.

In contrast, Skyhook maps out areas before hand so the users don't have to upload information if not inclined. Microsoft has also been rumored to be in the early stages of creating a similar initiative. Navizon is different, Houri says. Because our system is based on a peer-produced database, its always very dynamic and up to date. We have people mapping their areas constantly in most major metropolitan areas, and in more countries than we can count, which makes it a very good snapshot of the Wi-Fi landscape at any given point. Thats as opposed to other companies that are sending trucks out to do the mapping. Two years from now, [those databases] will be very inaccurate.

Anybody can register and download the free Navizon software for personal use. As long as they have a PPC with built-in a Wi-Fi and/or cellular radio, and as long as someone with a GPS has mapped the Wi-Fi and cellular landscape where they are, theyll be able to get accurate positioning.

Peer-developed databases are all very well of course as long as the peers actually do the tasks required  in this case keeping their PPCs on and receiving GPS and Wi-Fi and/or cellular signals, and then synchronizing with the Navizon server. How dense is the map of AP and cellular tower locations? It various from city to city and region to region.

In urban areas, its perfect positioning thanks to the density of access points you find there, Houri says. In suburban areas, its good enough. In rural areas, you might have a problem. This is why we also have cellular positioning. Its one of the cool things about Navizon  that it combines GPS, Wi-Fi and cellular positioning.

Registering as a user at the Navizon site, which you can do for free and without downloading the PPC software, gives access to the Google Maps the Navizon software uses. You can define a region by clicking at opposite corners of an imaginary rectangle and see a display of all the known APs and towers in that area. Their locations show up as map pins  red for GSM, green for Wi-Fi.

Major cities such as New York have been well mapped. My mid-size Canadian city appears to have enough sites mapped in a few areas to provide adequate positioning, but not everywhere.

Navizon doesnt stop at basic positioning. It offers some cool location based applications as well and is promising more. You can find your own position if youre in a strange place, for example  though not in real time, you have to click a button to download location data. Through Google mapping Navizon provides directions to points of interest such as the nearest Starbucks.

Users can already set up Buddy Tracker features to display the positions of their buddies on the Google maps. This has more serious applications, Houri says. Parents could use it to track family members, and businesses to track vehicles or employees.

A new application will let Navizon members post geotags  comments linked to particular locations, such as restaurants. Members will be able to subscribe to interest areas  restaurants, attractions, indie music  and will only see geotags related to their interests.

Navizon doesnt currently generate any significant revenues for Mexens, but the plan is to eventually charge business users. If a limousine company wanted to use Navizon to track its fleet in a big city where GPS service was too unreliable for the purpose, it would have to pay Mexens for a commercial software license.

Houri also hopes to license it to software developers so they could bundle it with offerings such as city guides. Users could not only read about a restaurant or attraction but also see its location and get directions from where they are. No company is paying Mexens for either type of license, yet, though the company has had approaches from prospective customers.

The business part of Navizon is mostly aimed at the future, Houri says. Were planning to start generating significant revenues in three to four months.

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